Currently, human liveness detection can be done by the non-invasive measurement of blood flow in a body. The methods are based on the well known Doppler effect—change of the wave frequency after reflection from a moving object. In practice ultrasonic, optical, or radar waves are used. In the case of ultrasonic or optical waves, after penetration into human skin the wave is scattered by static tissue as well as by movement of cells in blood vessels (platelets and others). While the former will stay unchanged compared to the source, the latter will be shifted in the frequency by a value proportional to the speed of scattering blood cells. As a result the scattered wave will have a broader spectrum and this effect can be measured and converted into the blood flow speed. Much of the prior art uses Doppler blood flow measurement methods and apparatus. Some of the methods use ultrasonic waves and some use laser radiation. Still other prior art uses Doppler radar measurement or detection of a beating heart or body part motion that is associated with breathing, i.e., respiration. In the Doppler radar case, a radar pulse is transmitted, and scatters from a breathing body or penetrates the body and scatters off a beating heart or moving body parts such as lungs and chest cavity, which move in the course of breathing, and the Doppler frequency shift of the reflected signal is sensed or measured. Additionally, some prior art employs thermal sensing of body temperature as an indication of liveness. One method of such thermal sensing is the detection of long wave infrared emission by a warm body.
As blood flow, breathing, and a beating heart exist only in live subjects the above methods can be in principle used for liveness detection. However, direct application of the known non-radar methods and apparatus to human liveness detection in combat or rescue operations is not feasible for several reasons. Ultrasonic Doppler velocimetry works only in proximity of a subject, preferably requiring contact between a transducer and a detector with the body, because the signal gets reduced dramatically with increased the distance from the body. The laser Doppler measurements do not require contact, but are still limited in distance. In addition, the laser source has a recognizable signature and can be detected by the enemy, which is not acceptable for covert operations. Also, for Doppler measurements a laser with very narrowband radiation must be used, as the speed of blood cells is slow and high precision measurement is necessary. Such a laser adds significantly to the cost and weight of the instrument. Pulse Doppler radar measurements are used in combat and emergency situations, but they are active measurements as they employ electromagnetic emissions that scatter from the subject. As such, the fraction of emitted power that is returned to the sensor is proportional to the inverse fourth power of the distance, i.e., the range, between the transmitter and the subject. As a consequence, such liveness detection requires substantial emitted power or energy and comprises a significant signature that can be detected by an enemy or if the power is reduced, then significant limitations in range result.
There are many multi-spectral and hyperspectral sensors and methods for a variety of physiological measurements, but these sensors and methods rely on close proximity or contact with tissue for such measurements. Such sensors and monitors are cumbersome and not well suited for remote sensing. Sensors that use long wave infrared detection of body temperature are not reliable when ambient temperature is near body temperature. At extreme temperatures, heat flow models must be used to account for the departure of skin temperature from normal body temperature. As a result, such sensing may be unreliable, require cumbersome calibration, or be useful only in limited conditions of ambient temperature and wind. Further, such sensors typically have poor spatial resolution and so rely on close proximity.
The ability to remotely determine if a wounded soldier was alive or dead would be of substantial benefit to battlefield leaders who must decide whether to risk personnel in recovering a soldier who might already be dead and thus unfortunately beyond medical help no matter how fast that help is provided.
The determination of liveness would also be useful to assess whether a still person poses a threat or to detect persons who are difficult to detect visually because they are in a visually cluttered scene or partially concealed. The detection and classification of pixels in a scene that correspond to live skin or tissue are useful for the detection of the use of cosmetics, i.e., ‘make-up’, prostheses, masks, or other means of concealment or alteration of appearance or disguise. With other indication of liveness such as locomotion or Doppler sensing of blood flow, breathing, or heartbeat, the identification of skin or tissue as not live would indicate an altered appearance that may be part of a disguise.